Parthenogenesis:
A form of reproduction where an egg develops without the fusion of sperm with the egg cell. Parthenogenesis occurs commonly among insects and other arthropods. Artificially inducing parthenogenesis with human eggs may be a means to isolate stem cells from an embryo, without fertilization.

Pentosan Polysulfate:
Pentosan polysulfate, PPS, is used to relieve bladder pain and discomfort related to interstitial cystitis, a disease that causes swelling and scarring of the bladder wall. Pentosan polysulfate is similar to a class of medications called low molecular weight heparins. It works by preventing irritation of the bladder walls. It comes as a capsule to take by mouth. It is usually taken with water three times a day, 1 hour before or 2 hours after meals. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take pentosan polysulfate exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.

Peptide:
A peptide is a chemical compound containing two or more amino acids (amino acid polymers) that are coupled by a peptide bond. This bond is a special linkage in which the nitrogen atom of one amino acid binds to the carboxyl carbon atom of another.

Perimysium:
A sheath of connective tissue that groups muscle fibers into bundles (anywhere between 10 to 100 or more) or fascicles.

Advances in muscle physiology suggest that the perimysium plays a role in transmitting lateral contractile movements. This hypothesis is strongly supported in one exhibition of the existence of "Perimysial Junctional Plates" in ungulate Flexor carpi radialis muscles constructed by E Passerieux. The overall comprehensive organization of the perimysium collagen network, as well as its continuity and disparateness, however, have still not been observed and described thoroughly everywhere within the muscle.

Phenotype:
The description of the characteristics of a cell, a tissue or an animal; as black and white fur of a mouse are two phenotypes that can be found. The phenotype is determined by the genes (or the genotype) and by the environment. For example, short stature is a phenotype that can be genetically determined (and therefore inherited from the parents), but can also be caused by malnourishment during childhood (and therefore be caused by the environment).

Phylogeny (phy·log·e·ny:
1. The evolutionary development and history of a species or higher taxonomic grouping of organisms. Also called phylogenesis.
2. The evolutionary development of an organ or other part of an organism: the phylogeny of the amphibian intestinal tract.
3. The historical development of a tribe or racial group.

Plasticity:
A phenomenon used to describe a cell that is capable of becoming a specialized cell type of different tissue. For example, when the same stem cell can make both new blood cells and new muscle cells.

Pluripotent Stem Cells:
Having the ability to give rise to all of the various cell types of the body. Pluripotent cells cannot make extra-embryonic tissues such as the amnion, chorion, and other components of the placenta. Scientists demonstrate pluripotency by providing evidence of stable developmental potential, even after prolonged culture, to form derivatives of all three embryonic germ layers from the progeny of a single cell and to generate a teratoma after injection into an immunosuppressed mouse.

Post-implantation Embryo:
Implanted embryos in the early stages of development until the establishment of the body plan of a developed organism with identifiable tissues and organs.

Precursor Cells:
A population of cells in the bone marrow that are incapable of self-renewal; they are derived from progenitor cells and are the first cells of a particular blood cell line that can be recognized due to their specific morphologic characteristics.

Pre-implantation Embryos:
Fertilized eggs (zygotes) and all of the developmental stages up to, but not beyond, the blastocyst stage.

Progenitor:
A cell that originates from a stem cell and differentiates into a more specialized cell.